[api] Collapse the three protobuf decode virtuals into one

Every decodable message overrode up to three virtuals, one per wire
type, so each carried a five slot vtable and up to three functions
with their own prologue and return tails. The shared decode loop now
parses the payload for the wire type into a ProtoFieldValue and calls
a single decode_field() virtual with the tag, the field number and the
wire type; the generated override is one switch.

The switch key is chosen per target through PROTO_DECODE_KEY. Embedded
builds compile switches to compare chains (ESP-IDF passes
-fno-jump-tables), so they key on the full wire tag, one compare per
field with no separate wire type check. The host compiler builds a
jump table for the dense field number switch, so there the key is the
field number and PROTO_DECODE_GUARD rejects a mismatched wire type.
Both forms drop a field that arrives with a wire type it does not
declare, exactly as the per wire type virtuals did.

Per decodable message the vtable shrinks from 20 to 12 bytes on
xtensa and the extra decode functions fold into one; the shared loop
shrinks as well. Host instruction counts per decoded field are
unchanged apart from the guard compare, which replaces the prologue of
the separate function it used to call.

tests/unit_tests/components/api/test_api_protobuf_generator.py

@@ -18,7 +18,9 @@ sys.path.insert(0, str(Path(__file__).parents[4] / "script" / "api_protobuf"))
 import aioesphomeapi.api_options_pb2 as pb  # noqa: E402
 from api_protobuf import (  # noqa: E402
     MAX_MESSAGE_ID,
+    SOURCE_CLIENT,
     _make_ifdef_line,
+    build_message_type,
     create_field_type_info,
     get_varint64_ifdef,
     validate_message_id,
@@ -182,3 +184,68 @@ def test_multi_byte_tag_fixed32_falls_back_to_the_generic_helper(
     content = _encode_field(field_type, number=16)
     assert "write_tag_and_fixed32" not in content, content
     assert content.startswith("pos = ProtoEncode::encode_"), content
+
+
+def _decode_cases(field_type: int, number: int) -> list[str]:
+    """Return the decode_field() case lines the generator emits for one decoded field."""
+    field = descriptor_pb2.FieldDescriptorProto(
+        name="value", number=number, type=field_type
+    )
+    ti = create_field_type_info(field, needs_decode=True, needs_encode=False)
+    return [
+        case
+        for case in (
+            ti.decode_varint_content,
+            ti.decode_length_content,
+            ti.decode_32bit_content,
+        )
+        if case
+    ]
+
+
+UINT32_T = descriptor_pb2.FieldDescriptorProto.TYPE_UINT32
+STRING_T = descriptor_pb2.FieldDescriptorProto.TYPE_STRING
+BOOL_T = descriptor_pb2.FieldDescriptorProto.TYPE_BOOL
+
+
+@pytest.mark.parametrize(
+    ("field_type", "number", "wire_type", "accessor"),
+    [
+        (UINT32_T, 2, 0, "value.as_varint()"),
+        (BOOL_T, 3, 0, "value.as_varint() != 0"),
+        (STRING_T, 1, 2, "value.data()"),
+        (FLOAT, 4, 5, "value.as_float()"),
+        (FIXED32, 5, 5, "value.as_fixed32()"),
+    ],
+)
+def test_decode_cases_carry_field_number_and_wire_type(
+    field_type: int, number: int, wire_type: int, accessor: str
+) -> None:
+    """Each decoded field yields one case keyed on its number and declared wire type."""
+    cases = _decode_cases(field_type, number)
+    assert len(cases) == 1, cases
+    lines = cases[0].splitlines()
+    assert lines[0] == f"case PROTO_DECODE_CASE({number}, {wire_type}):", cases[0]
+    assert lines[1].strip() == f"PROTO_DECODE_GUARD(wire_type, {wire_type});", cases[0]
+    assert accessor in cases[0], cases[0]
+
+
+def test_message_gets_a_single_decode_field_override() -> None:
+    """All wire types of a decoded message land in one decode_field() switch."""
+    desc = descriptor_pb2.DescriptorProto(name="Mixed")
+    desc.field.add(name="name", number=1, type=STRING_T)
+    desc.field.add(name="count", number=2, type=UINT32_T)
+    desc.field.add(name="level", number=3, type=FLOAT)
+    header, cpp, _ = build_message_type(desc, {}, {"Mixed": SOURCE_CLIENT})
+    decl = "bool decode_field(uint32_t tag, uint32_t field_id, uint32_t wire_type, ProtoFieldValue value) override;"
+    assert header.count(decl) == 1
+    assert "decode_varint" not in header and "decode_length" not in header
+    assert (
+        cpp.count(
+            "bool Mixed::decode_field(uint32_t tag, uint32_t field_id, uint32_t wire_type, ProtoFieldValue value) {"
+        )
+        == 1
+    )
+    assert "switch (PROTO_DECODE_KEY(tag, field_id)) {" in cpp
+    for number, wire_type in ((1, 2), (2, 0), (3, 5)):
+        assert f"case PROTO_DECODE_CASE({number}, {wire_type}):" in cpp, cpp